MOS (metal oxide semiconductor) types of transistors have progressively become smaller and smaller, and accordingly so have the gate electrode structures of the MOS-type transistors. Such reductions in size have been achieved by, among other things, evolutionary changes in gate electrode structures. Occasionally, a first change that was made to reduce the gate electrode size has caused a subsequent second change that compensated for an unintended consequence of the first change.
Consider an example of such first and second changes in the context of a gate electrode structure having a substrate, on it a gate insulating layer, and on it a polysilicon layer. As a first change, a metal layer was used to replace the polysilicon layer in the gate electrode structure. During subsequent heat treatments of the revised gate electrode structure, the metal layer undesirably reacted with the gate insulating layer. As a second change that compensated for the first, a barrier layer was interposed between the gate oxide layer and the metal layer.